The earth's geologic history has been punctuated by five
massive extinctions. In scientific terms a "mass extinction" can be described as follows:
A mass extinction is any substantial increase in the amount of extinction (i.e., lineage termination) suffered by more than one geographically widespread higher taxon during a relatively short interval of geologic time, resulting in an at least temporary decline in their standing diversity [numbers of species].
In other words, mass extinctions rapidly affect many kinds of living things and are very widespread or even global in extent.
Major phylogenetic groups affected at each extinction pulse:
2. Devonian (365 mya) -- decimation of coral reefs, brachiopods, and calcareous foraminifera.
3. Permian (250 mya) -- estimated 96% extinctions at species level in the marine realm, and for the first time, drastic reduction in the number of terrestrial tetrapod families.
4. Triassic (210 mya) -- extinctions wiped out 23% of both marine and non-marine animal families, including sponges, gastropods, bivalves, cephalopods, brachiopods, insects, and vertebrates.
5. Cretaceous (65 mya) -- the extinction of all non-avian dinosaurs, plus substantial to complete losses among such diverse groups as ammonites, nannoplankton, rudists, and certain marine reptiles.
The end-Pleistocene extinction event does not qualify as a mass
extinction. It is better classified as a taxon-specific event,
affecting primarily the Class Mammalia (although birds and, to
a lesser extent, reptiles were also affected). Nor was it global,
although later in the Quaternary many other regions were affected
by dramatic losses of a similar sort.
Background extinctions are at the opposite end of the spectrum from
mass extinctions. It is generally assumed that "natural" extinctions
go on all the time, but at very low or "background" rates. Characteristic
of background extinctions is that they occur in a random or uncorrelated
manner. One high estimate for the recent background extinction
rate for birds is one species extinction per 400 years. If only this natural
rate of loss affected the number of bird species, no more than a couple
of extinctions should have occurred in the past 800 years. Scientists
estimate that the actual loss during this time period lies somewhere between
200 and 2,000! The fact that today's extinction rate vastly exceeds any
estimation of the background extinction rate impels many scientists to
conclude that we are now on the cusp of the so-called Sixth Extinction.